The invention relates to a composite of silver and at least two metal oxides manufactured by milling, sintering and compressing a powder thereof. The composite is useful as a contact layer of a two layer electrical contact, the other layer having good soldering or welding characteristics.
In many high current density applications, highly conductive metals, such as silver, are not suited for use as a material for electrical contact elements. The surface of such a metal will frequently become partially melted and splattered in the arc which is generated in switching processes, particularly if the contacts chatter when being closed. This leads to a large amount of material burn-off and also can cause the contact elements to weld together so that they can be separated only be application of a force (known as the welding force). As a result, such contact elements are typically hardened by addition of a metal oxide.
In particular, cadmium oxide has been used as a metal oxide component for hardening silver electrical contacts. This type of element has good heat resistive characteristics and does not exhibit material flow. However, while contact elements of silver and cadmium oxide are useful for large current carrying applications, they also are undesirable. Their manufacture requires use of cadmium and cadmium salts which are poisonous and make disposal of the manufacturing wastes difficult. Moreover, their use in large electrical devices causes cadmium environmental pollution and cadmium toxification of workers repairing such devices unless special precautions are taken.
Nevertheless, contact elements of silver and a metal oxide other than cadmium oxide have not been developed which can meet electrical contact property specifications required for large electrical contact devices. For example, a contact element made of a silver and tin oxide dispersion exhibits in extruded form an unacceptably high temperature rise and material migration parallel to its extrusion dimension when it is used to carry large current. While excessive temperature rise can be reduced by addition of tungsten oxide (WO.sub.3), the material migration still occurs. It is only when a silver and tin oxide contact element carries current perpendicular to its extrusion dimension that practically no material migration will occur. This arrangement, however, cannot be used because a method for bonding or soldering the contact to its support has not yet been developed.